Abstract : At Château-Landon in 1910 or on the plateau Picard in 2001, the invasion of underground cavities by groundwater after heavy rain has caused surface subsidence and affected people and/or houses. To prevent these risks, INERIS studies the mechanisms of rupture in two underground chalk quarries partially flooded (Saint-Martin-le-Noeud and Estreux, northern France) where are measured the water level and the geomechanical parameters of a pillar (convergence, extension, saturation). At Saint-Martin-le-Noeud, the instrumented pillar is flooded and there is a gradient of water saturation of the rock which penetrates to at least 0.20 m deep and fluctuates depending on the level of flooding. At Estreux, the pillar is episodically flooded by groundwater whose level is situated a few meters down, but its water content increases by capillary with the water level: this causes both fluctuations of the pillar extension and roof-wall convergence.